This invention relates to apparatus for false twisting yarns, and in particular to apparatus of the type comprising three rotatable, parallel shafts located at the apices of an equilateral triangle, a plurality of parallel coaxial discs forming a disc stack carried by each shaft, the discs being arranged on the shafts so as to create a series of overlapping discs arranged on a helix around a line parallel with the shafts and centrally thereof. Such apparatus is well known and is described in, for example, U.K. Pat. Nos. 1379960, 1381132, 1419085, 1419086, 1419087, 1437464, 1456655, 1475698 and 2121076. The amount of overlap of the discs plays an important part in determining the level of twist which is imparted to a yarn as it passes through the apparatus and is false twisted by contact with the disc peripheries. U.K. Pat. Nos. 1379960, 1419085 and 1437464 describe arrangements whereby the spacing of the shafts may be adjusted whilst remaining equiangularly spaced. With such arrangements the shafts are driven by means of a drive belt which passes around a drive pulley and a wharve or pulley provided on each shaft. In order to accommodate the variable shaft spacing, the drive belt also passes around a belt-tensioning "jockey-wheel" mounted on a resiliently biassed pivotal arm. This arrangement leads to a variation in belt tension between opposite ends of the adjustment range and hence possibly to unsatisfactory drive transmission, undue belt stretching and/or premature belt failure. Furthermore such arrangements usually involve reverse bending of the drive belt, which also contributes to the shortening of belt life and to excessive consumption of power.
It is an object of the present invention to provide an apparatus of the abovementioned type which avoids the abovementioned disadvantages or is subject thereto to a lesser extent.